Breast milk is the best food for infants. However, lactation is not without risk to the mother. In addition to the relatively minor complaints of painful breasts and nipples, lactation can lead to lactation mastitis. This neglected condition has a clinical spectrum ranging from focal inflammation with minimal systemic symptoms to abscess and septicemia. Treatment may require intravenous administration of antibiotics and surgical drainage. Estimates of the incidence of lactation mastitis per infant breast-fed range from 3% to 9%; approximately 11% of these women will develop an abscess. Although clinical impression suggests it is relatively common and associated with inexperienced nursers and improper nursing techniques, few studies have described the incidence and even fewer the associated risk factors. Moreover, despite extensive literature on the physiology of lactation, recommendations for breast- feeding frequency, duration and positions, and discussion of maternal concerns and problems about breast-feeding in the neonatal period, we found no population-based studies describing how women actually breast- feed. We propose to follow a cohort of 1200 breast-feeding women enrolled at two sites for three months post-partum with the following goals: I. To describe initial breast-feeding practices, how these practices change over a three month period, and how and why these women wean their infants, among 600 breast-feeding women employed by a single organization and 600 breast-feeding women delivering at a freestanding birthing center. II. To determine the incidence of lactation mastitis in this sample, and to test the association of this incidence with breast-feeding practices, sleep deprivation, and stress. The prospective cohort study will consist of contacting all women from each site who do not decline participation for a short [10-15 minute] computer assisted telephone interview at 3 weeks, 6 weeks, 9 weeks, and 12 weeks post-partum. The interview will include questions on breast-feeding practices, mastitis, sleep deprivation, stress, and socio-demographic variables during the three weeks prior to each interview. For variables measured over time, the same questions will be used at each interview. Responses during the interview will be entered directly into the computer. Data will be analyzed to describe breast-feeding practices over time and to describe the associations of hypothesized risk factors with mastitis. The proposed study will take the first steps toward better understanding how women breast-feed and wean their infants, the risk factors for lactation mastitis, who is at high risk for mastitis, and what measures a high risk woman might take to decrease her risk of infection. This is the first controlled study examining the role of breast-feeding practices, stress and sleep deprivation on mastitis risk. If these factors are found to put women at higher risk of lactation mastitis, the results will have immediate clinical application.